Field of the Invention
The present invention relates generally to nanoporous films formed from a series of block polymers derived from cyclo-olefinic monomers and more specifically use of such films in pervaporation processes, water purification and as antireflective coatings.
Description of the Art
A wide variety of nanoporous organic materials have been reported in the literature having a broad spectrum of applications including separation membranes, battery separators, microelectronics and anti-reflective coatings. This is partly due to the fact that nanoporous materials especially in the form of films feature high pore density, low dielectric constant and low refractive index. Although the exact structure needed is driven by the intended application most sought after property being mechanical properties and retention of pore size, porosity and pore connectivity. For example, anti-reflective coatings require both high porosity (low refractive index) and small pore size (below diffraction limit). One challenge to fabricate nanoporous organic polymeric films with high porosity (>50%) is their relatively weak mechanical properties that lead to the collapse of the desired nanopore structure.
It has been reported in the literature that block copolymers (BCPs) can be used to form nanoscale structures via self-assembly with the size tunable by the molecular weight. For example, polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) and polystyrene-block-polylactide (PS-b-PLA) have been used to form such self assembled structures, where PMMA can suitably be removed by ozone, chemical treatment, see for example, U. S. Patent Application Publication No. US2011/0120970 A1. Also see, WO2012/035292 A2. Similarly, the lactide block in a PS-b-PLA has reportedly been removed by suitable hydrolytic degradation, see for example, Zalusky, et al., J. Am. Chem. Soc., Vol. 124 (No. 43), pp 12761-12773 (2002).
It has also been reported in the literature that solvent swelling can induce porous structure in block copolymers. For instance nanopores have been formed by swelling of block copolymers such as PS-b-PMMA or polystyrene-block-poly(2-vinyl pyridine (PS-b-P2VP). In the latter case for example the selective swelling of P2VP cylinders by ethanol leads to plastic deformation of majority PS phase to generate highly ordered nanopores after drying. See for example, Yin et al., ACS Nano, Vol. 7, pp 9961-9974 (2013). However, the porosity of the films so formed is low (<25%). Although the porosity could be increased by swelling the majority phase and using minority phase as the mechanical support during drying, the challenge is to prevent collapse of the swollen structure at least due to three main factors. First, the mechanical properties of the solvent swollen block copolymer may significantly be decreased compared to the neat copolymer. Second, the magnitude of the capillary force developed on evaporation of the solvent increases as the pore size decreases leading to a large applied stress. Third, as the porosity increase, the wall thickness decreases which further reduces resistance to deformation by external stress.
Accordingly, it is an object of this invention to provide a series of block polymers derived from polycycloolefinic monomers, more specifically, suitably functionalized norbornene monomers, which exhibit high glass transition temperatures, greater than 300° C., and high mechanical properties. The block polymers of this invention can also be tailored such that they can provide a significant solubility contrast for swelling.
It is also an object of this invention to provide processes for the generation of stable, high porosity films as disclosed herein.
It is further an object of this invention to provide high porosity nanoporous films which exhibit robust mechanical properties having a variety of applications including pervaporation membranes and antireflection properties, among various other uses.
Other objects and further scope of the applicability of the present invention will become apparent from the detailed description that follows.